Power devices that are low loss, compact, highly functional devices capable of operating with high current and high voltage are most promising as next-generation semiconductors. As development of such devices continues to advance in recent years, the requirements for sealing materials for Si, SiC or GaN devices have become ever more stringent. In particular, there is high demand for use in high-temperature environments, i.e., greater heat resistance, to allow driving at higher power. Relatively high heat-resistant polyimide resins, silicone gels and high heat-resistant epoxy resins have therefore been used in the prior art as sealing materials for power devices.
Polyimide resins (with glass transition temperatures of 350° C. or higher) have high heat resistance but also poor workability, requiring high temperatures and long time periods for molding. Silicone gels (with glass transition temperatures of 400° C. or higher, or else not observed) are used in potting-type molding and therefore require cases that can support their shapes during molding, and the resins themselves are expensive, rendering them disadvantageous in terms of cost and productivity. Heat-resistant epoxy resins (glass transition temperatures of 100 to 200° C.) have excellent workability but are inferior to the aforementioned two types of materials in terms of heat resistance, including high-temperature mechanical properties and electrical characteristics. Moreover, since heat-resistant epoxy resins have special structures with a naphthalene backbone or tetraphenyl backbone, they are costly and limited in their practicality.
On the other hand, thermosetting resins with high heat resistance are known, namely compositions including an alkenylphenol compound and an aromatic bismaleimide compound (for example, see Patent Document 1 (Japanese Unexamined Patent Publication No. H5-43630) and Patent Document 2 (Japanese Unexamined Patent Publication No. H6-93047)). A cured resin with high heat resistance can be obtained by radical polymerization between the alkenyl groups of the alkenylphenol compound and the unsaturated groups of the aromatic bismaleimide compound, to produce a high degree of crosslinking. Such a thermosetting resin (having a glass transition temperature of 200 to 350° C.) has heat resistance that is inferior to that of a polyimide resin or silicone gel, but still exhibits higher heat resistance than a heat-resistant epoxy resin while also allowing transfer molding similar to epoxy resins, and therefore such resins are known to exhibit both heat resistance and molding workability.
However, since compositions including alkenylphenol compounds and aromatic bismaleimide compounds have higher crosslinking point density than epoxy resins, the impact resistance of their cured products are low due to a high flexural modulus, and therefore when cured they are hard and brittle. Moreover, when the compositions are used as sealing resins, because of the phenolic hydroxyl group in the alkenylphenol compound that remains without contributing to polymerization, the humidity resistance of the sealed apparatus may be impaired.
Patent Document 3 (Japanese Unexamined Patent Publication No. H5-6869) discloses a semiconductor device having a semiconductor element sealed with a resin composition including a special phenol resin with an alkenyl group and hydroxyl group of which ratio being in a specified range in the molecule (phenolaralkyl (ZYLOCK) resin), a maleimide compound having two or more maleimide groups per molecule, and a curing catalyst. Patent Document 3 describes that such a resin composition can be used to obtain a semiconductor device having excellent humidity resistance, heat-resistant reliability and high reliability of mechanical strength.
However, because of the complexity of its synthesis, ZYLOCK (phenolaralkyl) resin is commercially supplied as a special grade not suited for general purpose use, and its cost is high. In addition, although Patent Document 3 discloses the flexural modulus, flexural strength and water absorption ratio when ZYLOCK resin is used as the phenol resin and an organic phosphine compound is used as the curing catalyst, absolutely no concrete examples are provided for using other phenol resins and curing catalysts.